PHP's documentation allowed (allows? Not sure anymore) comments. Especially in the pre-StackOverflow days, this was absolutely brilliant. I'm sure that's a pretty heavy moderation burden for any successful language (especially these days, maybe not as much when PHP's docs first added that feature) but damn it's nice.

I love the idea of bugtracker-as-forum (you're right that they already are and we're just in denial about it and don't design them to do that well) but it's the kind of thing that doesn't seem productizable (how many companies would want that, for instance? I doubt many) and non-commercial non-trivial open source isn't such a lively field, lately.